Why is usable capacity lower than the rated capacity?

Two separate losses stack: the chemistry is not discharged all the way down, and converting stored direct current into a mains-style output costs energy on the way out. The usable figure is what reaches the socket, and not every maker publishes it.

A battery holding a stated number of watt-hours does not deliver that number, and the gap is bigger than most listings admit.

The reserve the chemistry keeps

Discharging a cell completely shortens its life, so the management system stops before that point. On some chemistries the reserve is a small fraction; on lead-acid the conventional working depth is far shallower still. The nominal rating counts the whole pack; the usable rating counts what you are allowed to take.

The cost of conversion

Producing an alternating waveform from a battery is a conversion, and every conversion has an efficiency. Makers who publish one usually quote a peak figure, which occurs at a particular load rather than across the range.

The load that is always running

An inverter left switched on consumes something even with nothing plugged in. Over a weekend, that idle consumption can be the largest single line in the budget on a small system — which is why a switch, or an eco mode, is worth more than it looks.

How to plan around it

Size from the usable figure where a maker publishes one, and treat the nominal figure as a comparison number between products rather than a promise about runtime. Where only the nominal figure exists, this site says so on the product page rather than converting it silently.

Where this page and the manufacturer’s documentation differ, the documentation is the one to follow. Every capacity, output, input window, current limit, cycle count and temperature threshold on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And the distinction this page keeps: a nominal capacity is not the energy that reaches a socket. A reserve the chemistry never gives up, and the cost of converting stored direct current into a mains-style output, both sit between the two. Where a maker publishes a usable figure, this page reports it beside the nominal one; where only the nominal one exists, it says so rather than converting it.